A quasi-synoptic view of the frontal circulation in the Crozet Basin during the Antares-4 cruise

With the major objective of estimating the secondary vertical circulation, a fine-grid survey of hydrographic and current fields across the Crozet Basin frontal zone was made as a physical component of the multidisciplinary Antares-4 cruise. This frontal zone forms the boundary of the Antarctic Circumpolar Current and the Agulhas Return Current, with the quasi-permanent existence of strong eddy activity. Prior to any quantitative diagnosis of the vertical circulation from the fine-grid data, it is necessary to place the mesoscale circulation and its time variability of the survey area within the basin-wide context. We give a quasi-synoptic view of the ambient frontal circulation during the cruise period, using satellite-derived chlorophyll concentrations and sea-surface temperature. Particular emphasis is placed on evaluation of the degree of synopticity of our fine-grid measurements by comparing mesoscale features on satellite images. The limited survey area (1.5° latitude×2° longitude) lies precisely within the southernmost and easternmost convergent zone of the two current systems, cutting across mesoscale meanders having typical wavelengths of 400 km and periods of 4 months. Both these satellite images and TOPEX/POSEIDON sea-surface height data indicate little temporal variability of the survey area during the 14 days of measurement. Although fortuitous, this quasi-steady state of circulation enables us to use the ensemble of fine-grid data as a set of synoptic data for diagnosing the vertical circulation using quasi-geostrophic dynamics. Preliminary analysis of the horizontal and vertical hydrographic structure of the fine-grid hints at the existence of wave-like cross-frontal intrusions having a mean along-front wavelength of about 30 km.